1. Field Of The Invention
This invention relates to projection printing of multi-color pictures from transparencies. More particularly, it relates to an attachment for a standard type photographic enlarger, including a plurality of filters and control means for sequencing the filters into and outfrom the light path of the enlarger, to provide three separate exposures in sequence for the colors red, green and blue, and means for establishing the length of each exposure as a function of the density of the transparency in the enlarger.
2. Description Of The Prior Art
The basic technique of producing color photographs by projection printing is well known and need not be described in detail.
There are two common ways of controlling color balance and exposure in a projection printing process. The first involves making a single exposure by use of a white light that has been modified by substractive filters. The second involves making a sequence of three separate carefully calculated exposures for the three basic colors red, green and blue.
An advantage of the single exposure technique is that the total exposure time is relatively short since all three of the color sensitive emulsions on the photographic paper are exposed at the same time. This provides for high volume production but experience has shown that it is very difficult to control color balance. The three filters are inserted only partially into the path of the white light. The portion of the white light that is modified in this manner is mixed in a mixing chamber with the remaining unmodified white light and then the mixture is directed through the lens to the photographic paper. Most enlargers using this type system include a manual setting for each filter by which the operator controls the area of each filter which is placed in the path of the white light. It is quite difficult to accurately set the filters in this manner. It usually involves a time and material consuming trial and error process for each negative. Negative color analizers are available but their use is also time consuming and the results are not always good because it is not always possible to have a known reference area from which to take a color density reading. Also, it is difficult to relate a reading to a specific enlarger, or printer. Some enlargers are available which include automatic controls which perform well but they are very expensive. A further problem of the single exposure technique is that accurate settings become more difficult as the bulb ages and the quantity and quality of the light decreases.
Exposure times of long duration are customary and preferred in the art of making large photographic enlargements. Long duration exposure times and inexpensive and accurate automatic control are possible if three separate exposures are employed, one for each of the basic colors red, green and blue.
A principal object of the present invention is to provide a relatively simple, inexpensive, but workable automatic control system for an enlarger, based on the use of three separate exposures for the colors red, green and blue.